Presently, most circuit protective devices (single phase or multi-phase) are constructed in a manner that if an over-current condition should occur in a phase, the mechanism will automatically open all phases. In most cases, fault detection will bring about automatic tripping in an "instantaneous" or a "time delay" basis. The "time delay" tripping occurs when the level of over-current is not excisable high, and is an action which is controlled by a bimetallic member heated by energy which is a function of the current flowing through the particular phase in which the bimetallic member is located. Typically, the bimetallic is disposed adjacent to a conductor and is intended to sense heat. The heat deflects the bimetallic and as the bimetallic deflects a predetermined distance, it engages a trip mechanism causing contacts to separate and interrupt the flow of current in the circuit breaker.
An individual bimetallic member is provided for each phase of a circuit protective device. One conventional calibration arrangement for "time delay" tripping provides for a calibration screw mounted directly on the trip bar or on the bimetallic member itself. During the calibration process, the actuation of this calibration screw provides the means of creating the appropriate distance in between the bimetallic member and the trip mechanisms to allow the bimetallic member to deflect when heated by energy for a period of time before tripping the phase.
However, this process of calibration and particularly, the adjustment of the calibrating screw, often applies additional load to the tripping means. This force introduces uncontrolled loads providing inconsistency and unpredictable results in the operation of the trip mechanism. Also, calibration varies due to changes in trip bar loads or in calibration forces.
In addition, it is very difficult to select and feed calibration screws reliably in automation because usually the features at both ends of the headless screw are very similar. Yet one end serves a completely different functional purpose in the calibration assembly. For example, most calibration screws for circuit protective devices have a rounded end at one end to serve as a pivot point and a drive feature, such as a slot or socket, at the opposite end to permit the calibration screw to be manipulated. Because both ends are very similar in terms of screw shape and cross section, the automation device has a hard time consistently detecting the front of the calibration screw from the back. As a result, sometimes, the calibration screw is inserted backwards.
Calibration of a molded case circuit protective device provides the means to set the device so that the device meets the manufacturer's advertised trip curves of percentage rated current vs. time. A common method of calibrating a circuit protective device is called cold calibration. This process permits for adjusting the position of the calibrating screw to the bimetallic member without the initial utilization of current (to deflect the bimetallic member) being applied to the phases as a means to set the bimetallic member to trip unit distance (gap). In the cold calibration process, the device under test must be first set to an "ON" position where the mechanism is charged and the breaker contacts are closed to provide a continuous path from the line to the load end of the device. The calibration s screw is then engaged and driven until it actually trips the mechanism, thus opening the contacts and braking the continuous path. Usually, upon actuation of the trip mechanism, the circuit protective device will trip and a pair of separable contacts will separate causing the current interruption in the circuit protective device. At this point, the calibration screw is reversed for a preset number of turns (the distance is based on the amount of threads per inch on the calibration screw). At this point, a distance is set and the initial cold calibration process must be now rechecked by charging the trip mechanism back to the "ON" position and applying a percentage of the rated current to the phase(s) until the unit trips as a result of bimetal deflection. Typically, the percentage of rated current applied to the device at this point will be from about 200% to about 400%. The time that elapsed for the circuit protective device to trip at this percentage current must fall within the parameters or "window" specified in the trip curves. If the device took too long to trip, an adjustment must be made to the calibration screw to adjust the present gap. An adjustment to the calibration screw must also be done if the device tripped too early. This adjustment will be done in the opposite direction. After this adjustment has been accomplished, usually the device is tested again to confirm the accuracy of the prior adjustments.
Another method utilized for calibration of circuit protective devices is the so called hot calibration process. The hot calibration differs from cold calibration because it puts the device under current conditions throughout the process. The standard practice for hot calibration provides for a calibration screw that is physically propositioned with respect to the bimetal, permitting for a gap that represents a particular time window. Again, as in cold calibration, the hot calibration process must start with the device in the "ON" position and the trip unit charged. A percentage of the rated current, typically about 200% to about 400% is applied to the device. At this time, a clock is also started providing a running count of the time that the current has been applied to the device. When the running time approaches the calibration window at which the circuit breaker must trip (current vs. time parameter for the particular rating), the calibration screw is driven quickly towards the deflecting bimetal. Once the calibrating screw makes contact with the incoming bimetal, the device will trip and the screwdriver will not be driven any further. Once the device opens and the bimetal cools down and returns to its original stable position, there is a gap in between the bimetal and the calibration screw. Once the percentage of rated current is applied to the circuit protective phase device again, this time for the bimetal to deflect and trip, the mechanism must fall within the correct window of time set for its rating. As in cold calibration, if when tested, the time period exceeds or is below the time allowed for tripping, further adjustments to the calibration screw are required.
Both hot and cold calibration processes have a variety of parameters that must be monitored and controlled to properly calibrate the circuit protective device. For example, some of these parameters are the rate of speed at which the screw is driven, the ability to pick up the screw immediately, the forces that the driving screw and the driving tool exert on the bimetal or other acting members of the trip mechanism, bimetal flatness, electrical joint integrity due to weld process variations, and other factors.